The general structures and manufacturing processes for electronic packages, including the use of positive photoresists in the fabrication of electronic packages, are described in, for example, Donald P. Seraphim, Ronald Lasky, and Che-Yo Li, Principles of Electronic Packaging, McGraw-Hill Book Company, New York, N.Y., (1988), and Rao R. Tummala and Eugene J. Rymaszewski, Microelectronic Packaging Handbook, Van Nostrand Reinhold, New York, N.Y. (1988), both of which are hereby incorporated herein by reference.
As described by Seraphim et al., and Tummala et al., an electronic circuit contains many individual electronic circuit components, e.g., thousands or even millions of individual resistors, capacitors, inductors, diodes, and transistors. These individual circuit components are interconnected to form the circuits, and the individual circuits are further interconnected to form functional units. Power and signal distribution are done through these interconnections. The individual functional units require mechanical support and structural protection. The electrical circuits require electrical energy to function, and the removal of thermal energy to remain functional. Microelectronic packages, such as, chips, modules, circuit cards, circuit boards, and combinations thereof, are used to protect, house, cool, and interconnect circuit components and circuits.
Within a single integrated circuit, circuit component to circuit component and circuit to circuit interconnection, heat dissipation, and mechanical protection are provided by an integrated circuit chip. This chip is referred to as the "zeroth" level of packaging, while the chip enclosed within its module is referred to as the first level of packaging.
There is at least one further level of packaging. The second level of packaging is the circuit card. A circuit card performs at least four functions. First, the circuit card is employed because the total required circuit or bit count to perform a desired function exceeds the bit count of the zeroth and first level packages, i.e., the chip and chip modules. Second, the circuit card provides for signal interconnection with other circuit elements. Third, the second level package, i.e., the circuit card, provides a site for components that are not readily integrated into the first level package, i.e., the chip or module. These components include, e.g., capacitors, precision resistors, inductors, electromechanical switches, optical couplers, and the like. Fourth, attachment to the second level package provides for thermal management of the chip and module, i.e., heat dissipation. The primary purpose of the package remains electrical interconnection.
At various times in the life-cycle of a microelectronic package, it is necessary to reroute the electrical interconnection of the package, that is, to reroute wiring leads. This may be the result of engineering changes, for example to accommodate new chip sets. Alternatively, rerouting may be necessary to correct manufacturing defects, for example to provide alternative signal pathways around unintentional open circuits, or to take unintentional short circuits out of the circuit network.
Typically circuit rerouting is made by abrading or "cutting" surface circuitization on one side of the panel, and running jumpers on the surface of the package between vias and through-holes previously connected by the now abraded or "cut" wiring. Other methods of rerouting involve joining insulated jumper wires to appropriate surface features on the package or panel.
One method of rerouting interconnections is described in J. Canavan, Screen Printing Engineering Changes on PCBs: Polymer Thick Film Materials Replace Hand Wiring, Circuits Manufacturing, Vol. 19 (No. 11), pages 22-26, November 1979. This paper describes a screen printing system, utilizing low temperature thick film materials, to overlay new circuits on the surface of finished, but unpopulated, microelectronic circuit packages. As described by Canavan, the process, called "WINK" (wire ink), allows conductive circuitization to be "screened" on the panel over a previously applied dielectric layer. This dielectric layer insulates the pre-existing circuitry from the re-routed circuitry.
According to Canavan, the process begins with an inspection of all incoming boards, followed by "cutting" all of the surface circuitization paths that are to be rerouted, for example because of engineering changes or defects. An epoxy dielectric is applied to the surface, except for openings at pads and lands. Conductor paths are screened onto the cured dielectric layer. Next, 60:40 Pb:Sn solder paste is screened over the conductor paths. The process is completed by reflowing the solder.
Another rerouting process is described by S. Picard, Printed Circuit Card Rework Process, IBM Technical Disclosure Bulletin, Vol. 15 (No. 1), pp. 246-247, (June 1972). Picard describes a rework process which uses two plastic sheets of PTFE, a dielectric, soldered to the microelectronic circuit panel. According to Picard, existing conductive paths are eliminated, for example, by milling, and two sheets of dielectric are used to implement new conductive paths.
The first sheet, which carries no circuitization, only pads and lands, is laid on the microelectronic circuit panel. The second sheet, which carries circuitization on both surfaces is laid atop the first sheet. Both sheets have vias at locations corresponding to vias and lands on the microelectronic circuit panel. To define a new path, the needed vias on the circuitized sheet are "opened", and unneeded lands are eliminated, for example, by milling. All unneeded lands are eliminated from the uncircuitized sheet, except for those needed for reworks, changes, etc. The two sheets are aligned, and then soldered to the backside of the microelectronic circuit panel.
An alternative rerouting process is shown in A. F. Kozik, Reworking of Component Bearing Cards, IBM Technical Disclosure Bulletin, Vol. 20, No. 4, pp. 1532-1533 (September 1977). Kozik describes reworking a microelectronic circuit panel by removing unneeded components, breaking wired connections, and thereafter adding new components to the component bearing surface of the microelectronic circuit panel. The changes are made on what Kozik characterizes as a new printed circuit.
This new printed circuit is a sheet of a thin, flexible material, carrying a printed circuit pattern. The pattern is on the side of the flexible film facing away from the microelectronic circuit panel. The flexible film has a solder tab at each point at which it is to be bonded to a corresponding land on the microelectronic circuit panel, and soldered to the back surface of the panel.
An alternative method of rerouting is described in W. D. Saucier, Circuit Overlay Adapter Assembly For Circuit Changes, IBM Technical Disclosure Bulletin, Vol. 18, No. 7, pp. 2050-2051 (December 1975). Saucier describes reworking a microelectronic circuit panel by connecting the panel to a circuitization carrying overlay. The circuitization carrying overlay is a non-adherent, "stand-off" panel, connected to appropriate lands on the panel by elastomeric pins.
U.S. Pat. No. 4,438,561 to Richard W. Mueller for METHOD OF REWORKING PRINTED CIRCUIT BOARDS describes a method of connecting electrically unconnected conductors on a microelectronic circuit panel with conductors on a flexible circuit element. The patent describes forming a flexible circuit element having a non-conductive substrate with a predetermined circuit pattern thereon. The flexible element has at least two through holes.
This flexible circuit is aligned with respect to the circuit pattern, with the circuitization on the flexible element facing the pcb and being separated therefrom by an interleaved sheet of uncircuitized dielectric (element 24).
An electrically conductive path is then established between the previously unconnected elements on the microelectronic circuit panel via the circuitization on the attached flexible carrier.
A further alternative method of rerouting is described in U.S. Pat. 4,310,810 to Thomas Currie for REWORKABLE MULTI-LAYER PRINTED CIRCUIT BOARD. Currie describes multi-layer microelectronic circuit panels having a rework layer. The rework layer has a plurality of insulated wires, that is, wires that are insulated from each other and the microelectronic circuit panel. The wires each have a characteristic impedance, Z, that matches the impedance of an internal conductive plane.
The insulated wires can be semi-permanently attached to the rework layer. Semi-permanent adhesion is said to be achieved by a heat activated thermoplastic adhesive.
A further method of rerouting is described in U.S. Pat. No. 4,654,102 to Jean C. Wrey, Michel Jehay, and Andre Job for METHOD FOR CONNECTING PRINTED CIRCUIT BOARDS. In this patent preformed, high melting wire is formed, and deposited on the insulating panel.
Another method of rerouting is described in U.S. Pat. No. 4,731,704 to Anton Lochner for ARRANGEMENT FOR MODIFYING ELECTRICAL PRINTED CIRCUIT BOARDS. Lochner utilizes a non-conformal carrier, spaced from the microelectronic panel.
The above mentioned methods of rerouting wiring, repairing wiring, and making engineering changes fail to provide an inexpensive, low cost method of making engineering changes and repairs to microelectronic circuit panels, that can be carried out in the field, and are not particularly useful in making individual, customized engineering changes.